This application is a continuation-in-part of U.S. patent application having Ser. No. 006,990, filed Jan. 29, 1979 U.S. Pat. No. 4,254,466 entitled "Power Factor Monitoring and Control System for Resistance Welding".
This invention relates to a monitoring and control system for a resistance welding apparatus and, more particularly, to a control which senses a resistive change that occurs in the formation of a quality weld between the electrodes by monitoring a corresponding change in an interval of current present in a winding of a welding transformer to provide an automatic heat control for improving weld quality over the tip life as well as reducing power consumption or to terminate the weld current when a resistive change causes a preprogrammed change in the instant when a half cycle of weld current is extinguished. When resistance welding is used to weld two metal parts together, a number of parameters in the welding process must be monitored in order to provide a quality weld. One parameter which much be carefully monitored is the wear of the welding electrodes (tips) which must be compensated by increasing the weld heat to insure good quality welds throughout the tip life.
Prior art attempts which were used to assure weld quality despite tip wear (mushrooming) included the following monitoring (feedback) control techniques:
1. Optical (infrared), i.e., monitoring surface radiation to assess weld quality;
2. Weld expansion, i.e., monitoring electrode displacement (thermo-expansion);
3. Ultra-sonic, i.e., monitoring ultra-sonic transmissions transmitted through the weld area during the weld formation process;
4. Weld energy, i.e., monitoring weld energy during the formation of the weld;
5. Acoustic emissions (expulsion detector), i.e., monitoring the acoustic emissions occurring during weld formation; and,
6. Resistance change, i.e., monitoring the resistance change occurring during the weld formation.
Typically, the above prior art techniques of monitoring weld quality were implemented by attaching or positioning various fragile sensors and their respective leads in direct contact with the welding electrodes or in close proximity thereto. Unfortunately, these monitoring devices and their leads attached to or in close proximity to the welding electrodes were subject to breakage in industrial environments in which the welding apparatus is required to operate.
For the above-stated reasons, monitoring devices and their attendant leads attached to or in close proximity to the welding electrodes were met with disfavor and were rejected as a means of control because of continual maintenance problems. Examples of the above type of monitoring techniques and associated devices are contained in a publication entitled "Resistance Welding Control and Monitoring" published by the Welding Institute located at Abington Hall, Abington, Cambridge, CB 16A1, United Kingdom, copyrighted 1977.
One successful way to compensate for electrode wear without attaching monitoring devices next to or on the welding tips is found in the digital welder control system of U.S. Pat. No. 4,104,724 ('724 patent). The controller as described in this patent provides a maintenance interval counter and compensator control having a four-step stepper. The stepper control of this patent is used to automatically increase the weld heat after a preset number of welds are made by the electrodes based on past experience to compensate for electrode mushrooming. Moreover, the digital welder control system of the '724 patent is hereby incorporated by reference as a type of digital welder control that is particularly suited to be modified to incorporate the features of the present invention.